Using a stereo amp from a mono signal?

Im setting up a system in my basement and i wanted to use an older set of speakers my dad has. He built them in the 60s so im sure the drivers are shot. They still work but im sure they cant handle any kind of power.

What i thought ide do is keep them around because they look nice but replace the driver with a sub and just use them as sub cabnets.

The reciver i was going to use has a mono signal output, his receiver still works so what i thought was maybe i could use his receiver as the poweramp to drive the subs from the sub preamp of the receiver im going to use.

My question is, since his stereo has stereo in my output is mono can i just use an RCA splitter and just split the mono signal from my receiver out to his then just hook up the speakers as normaly?

When running a mono signal can i run the subs out of phase to get a better bass sound?

I remember when i messed around with car audio and i ran 2 subs i got way better bass when i ran them both out of phase from each other. (as one fired out when a bass note hit, the other woofer would pull in giving this feeling of more bass or somthing.

Will there be any ground loop problem or any feedback or somthing weird when i split the signal and send the same signal to both inputs?

Sorry i ramble sometimes.

My dad has some old speakers from the 60s he built but i wanted to keep them because he built them. You cant see the driver because of the grills he put on them. They look nice so i wanted to keep the cabnets but replace the driver for a sub. 2 nice old cabnets with 2 new subs.

Now my receiver im going to use with the surround sound format has all the ins and outs but it also has a pre sub output. What i thought ide do is keep his old receiver but use it as a power amp for the subs im gona put in his cabnets. Use his receiver as a sub power amp to power his cabnets with my new subs in it. They are a nice size for a 12" sub.

Since they were once full range home stereo speakers there is 2 so i thought insted of using a mono sub like most systems have use the stereo amp to power 2 of them

I was just wondering if ide have any problem running both inputs into 1 because my receiver just has the "mono pre sub out"

His receiver also has a 7 band EQ so i could evenremove alot of the high end in the signal if there is any

Well it was in a car, you know how tight cars are. The first setup i did this with the subs were in the same cabnet but ive had it happen with a devider also.

It was by accedent once and i jammed on it for a while then i rewired everything and found it lost alot of bass so i messed around with it and figured out what was going on. I then asked a car audio guy and he told me it was called "out of phase"

I havnt really been able to hear it work on home audio equipment. Either its somthing that only happens with DC or in small cars or it was just somthing that happened with amps back in the 80s and 90s.

I even remember amps with switched on them called reverse polarity that you could flip and hear the difference when you flipped it.

I do notice with home audio the bass sounds the best in the center directly between both speakers. With the out of phase setup it didnt do this. The bass was good anywhere.